Following newspaper reports that Theresa May is preparing to give ground in the discussions, McDonnell said the PM had “blown the confidentiality” of the talks.

He said his party needed guarantees that an agreement would not be “ripped up” by a future Conservative leader.

Asked on BBC One’s Andrew Marr Show if he trusted May, he replied: “No, sorry, not after this weekend when she’s blown the confidentiality I had and I actually think she’s jeopardised the negotiations for her own personal protection.”

The Sunday Times reported that May will suggest a temporary customs arrangement that would last until the next General Election. McDonnell said: “We are negotiating with Theresa May’s team as requested. Whilst we’re doing that and we think we’re gaining an understanding of our different positions and where we can reach some compromise, in the wings, if you like, are all the leadership candidates virtually threatening to tear up whatever deal that we do.

“So we’re dealing with a very unstable Government and let me just use this analogy – it’s trying to enter into a contract with a company that’s going into administration and the people who are going to take over are not willing to fulfil that contract. We can’t negotiate like that.”

McDonnell also said it “may well” be that any deal would have to be voted on in a second referendum, adding: “I think the Conservatives have to recognise that if a deal is going to go through there might be a large number of MPs who will want a public vote.”

During her appearance on the Andrew Marr Show, Scottish Tory leader Ruth Davidson insisted a deal with Labour is possible and her party should “starting walking ourselves back” from the “extremes” of the debate. She stated: “The two extremes at the edges are getting louder and louder and louder.

“On one side you’ve got people saying ‘this was wrong, let’s re-run the referendum, let’s overturn the result,’ and on the other side you’ve got people saying ‘let’s not do a deal at all, let’s just crash out’.

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